The glycoprotein hormone erythropoietin is an essential survival and growth factor for the erythrocytic progenitors in the bone marrow. Its peptide sequence of 165 amino acids is stabilized by 4 carbohydrate side chains (total molecular mass 30 kDa, 60% protein). Erythropoietin is synthesized mainly in the kidney. Its formation increases greatly in response to hypoxia. The mechanism by which lack of oxygen induces erythropoietin gene expression is only partly understood. Erythropoietin deficiency is the primary cause of the anemia in chronic renal failure. Erythropoietin production is lowered in severe acidosis and in inflammatory states. Genetical engineering has enabled the isolation of the human erythropoietin gene and its in vitro expression in mammalian cell cultures. Recombinant human erythropoietin has been approved to be an efficient and safe drug for treatment of renal anemia. This paper reviews some of the recent advances in the understanding of the biology of erythropoietin with respect to its possible application as an alternative to blood transfusion in surgical patients.